Downloaded more than 2.6 million times, this popular app promises that you can use it to start setting up your own online store in just minutes. Optional paid support and a hosted version called "PrestaBox" are also available. Operating System: Windows, Linux, OS X

Used by Second Life, Wayne Gretzky Estates, Coupon Trade and many other sites, Spree claims to be "the world's most flexible e-commerce platform." In addition to the free community version, it comes in Standard and Premium editions, which offer the same features but add paid support. Operating System: OS Independent

Designed to be mobile-friendly, Tomato Cart claims to be one of the most innovative open source e-commerce systems available. Key features include an RIA-based administration panel, search engine optimization, Web analytics, a content management system and a built-in email system. Paid support is available, as is third-party hosting. Operating System: Windows, Linux

This ecommerce solution was designed to make things easy for small business owners who are new to selling online. Extensive video and text documentation are available through the site, and it also provides links to service providers for payment processing, hosting and other related services. Operating System: Windows, Linux

Calling itself the "leading Cloud-based, open-source ERP software and customer relationship management (CRM) system," Compiere claims to be modern, adaptable and affordable. It comes in community and enterprise editions, as well as a cloud version that runs on Amazon Web Services. Operating System: Windows, Linux, OS X

Best for small businesses, freelancers and foundations, Dolibarr can be used on a standalone system or on a server for multi-user access. Several vendors offer cloud-based versions as well. Operating System: OS Independent

In addition to standard ERP features, this app also offers CRM, MRP, SCM and product design management. Commercial support and services are available through third-party partners. Operating System: Linux

Java-based JFire aims to be a highly flexible ERP solution and includes a development platform for those who want to customize the software themselves. Paid coaching, consulting and development services are available. Operating System: OS Independent

Designed for small to mid-sized businesses, Neogia offers a modular ERP with tools for tracking finance, e-commerce, manufacturing, sub-contractors, CRM, distribution, facilities, stock and e-commerce. The site includes an online demo so you can see it in action. Operating System: Windows, Linux

Used by more than 6,000 organizations around the world, Openbravo promises increased productivity, agility and return on investment. The community version is available for download on the website; the Professional Edition, Openbravo for Retail and the many commercial extension modules are available through Openbravo's retail partners. Operating System: OS Independent

This very comprehensive ERP solution comes with about a dozen modules, including CRM, accounting, point-of-sale and others, and users can also download more than 700 extension modules. In addition to the free community version, it also comes in a paid on-site enterprise version and a paid cloud version. Operating System: Windows, Linux

Opentaps counts Toyota, Honeywell and some other well-known companies among its users. Besides the free, downloadable software, the company also offers paid update services, a professional edition, consulting, training and more. Operating System: Windows, Linux

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